1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus provided with a detachable image bearing cassette having plural image bearing members.
2. Description of the Related Art
Up to now, in the field of image forming apparatuses, there has been proposed a so-called process cartridge detachable to an image forming apparatus, which is manufactured by integrating a photosensitive member. Also, there has been proposed an image forming unit that integrates plural photosensitive members and another image forming mechanism and is detachable to an image forming apparatus main body.
The above process cartridge and image forming unit are advantageous in that a user can easily perform replacement and maintenance without the help of a specialist. However, in such structure that the photosensitive members and the image forming mechanism are integrated, the entire process cartridge and the entire image forming unit should be replaced even if a user desires to replace a part of the photosensitive members and the image forming mechanism. Worse yet, in the case of the image forming unit having plural short-life photosensitive members, if any one of the photosensitive members needs to be replaced, the entire unit should be replaced even though the remaining members are serviceable. With regard to a color printer having four photosensitive members, the life of a black photosensitive member expires sooner than photosensitive members of the other colors due to frequent use. Thus, in order to replace the black photosensitive member, the entire image forming unit including the photosensitive members of the other colors should be replaced.
Further, in the process cartridge, a container of a cleaning device for rotatably supporting a photosensitive member is integrated with the photosensitive member. The cleaning container is provided with a process mechanism for charging the photosensitive member to form an image. The process mechanism refers to a charging roller for uniformly charging the photosensitive member, and a cleaning blade and cleaning roller for scraping off residual toner that remains on the photosensitive member even after transferring a toner image onto a sheet-like medium (hereinafter referred to as “sheet”). These are provided around the photosensitive member. The process cartridge can be attached/removed to/from an image forming apparatus, and replaced by a user himself/herself at the end of its useful life or when waste toner is fully filled.
In addition, the image forming unit incorporates plural photosensitive members, charging rollers and developing devices corresponding to the individual photosensitive members, and plural intermediate transfer drums as main components. The unit can be inserted/removed to/from the image forming apparatus, and replaced by the user himself/herself at the end of the useful life of the photosensitive members or intermediate transfer drums.
Meanwhile, there has been known a two-station recording system. In this two-station recording system, a developing device, a writing device, and a driving mechanism are supported to an apparatus main body via a common assembly member, and are positioned with high accuracy. According to such structure, the developing device is positioned relative to the apparatus main body and thus used as a reference position (assembly reference) for all components of an image forming process. The photosensitive member (or photosensitive unit) is incorporated into the developing device, not the apparatus main body. That is, the photosensitive member (or photosensitive unit) is positioned only relative to the developing device, so a relationship between the developing device and the photosensitive member (or photosensitive unit) is a master-servant relationship. The photosensitive member (or photosensitive unit) is detachable to the developing device, and the developing device is detachable to the image forming apparatus. Further, the photosensitive drum, the charging mechanism, and the cleaning mechanism are integrated together.
In recent years, stringent market requirements have been imposed on an image forming apparatus such as a printer or a copier. Accordingly, an image forming unit has become more important. Furthermore, it is known that the market requirements place importance on the photosensitive member out of the image forming mechanism. A detailed description thereof is given below from the viewpoints of three factors.
A first factor is reduction in diameter in photosensitive member accompanying downsizing of image forming apparatus. In recent years, market requirements have been directed toward downsizing an office machine, and accordingly, a downsizing is required of an image forming unit. However, if a drum-like photosensitive member as an image bearing member is reduced in size, that is, in diameter, a wearing degree per sheet is increased under the same image forming conditions. For example, assuming that the photosensitive diameter is reduced from 120 mm to 40 mm, the 40-mm photosensitive member needs to rotate at a speed three times higher than that of the 120-mm photosensitive member in order to form an image of the same size. This trebles various types of wearing of a photosensitive member upon image formation, such as an electrical wearing due to discharge etc. in a charging unit or a mechanical wearing due to a blade in a cleaning unit. Hitherto, a developing device or other such devices have been downsized to some degree. However, size reduction of a photosensitive member has not been pursued as actively as other image forming units, from the viewpoint of avoiding the above wearing. Nevertheless, it is inevitable that the photosensitive member will be downsized to meet an increasing requirement for size reduction. As mentioned above, the requirement for size reduction places much weight on the photosensitive member and shortens its useful life.
A second factor is slimness of photosensitive member along with high image quality. In recent years, users have output photographic images or graphic documents more than before. Accordingly, an attempt to increase an image quality up to a film image has been made. A typical way to increase the image quality is increasing a resolution. However, a slim photosensitive member is required for increasing the resolution in an electrophotographic process. For example, in a negatively-charged photosensitive member, carriers generated in a CGL (charge carrier generation layer) pass through a CTL (charge carrier transport layer) to reach the photosensitive member surface to thereby form a latent image on the photosensitive member. If the CLT is thick in this case, the carriers should move for a long distance, with the result that the carriers are separated from one another due to electrical repulsion between the carriers. If so, a latent image corresponding to a writing signal cannot be formed, resulting in an image dot positions of which are a little incorrect. As a result, an image quality is deteriorated. This problem arises not only in the case of increasing the resolution of the electrophotography from 600 dpi to 1200 dpi but also in the case of improving an image quality with the resolution kept at 600 dpi, for example, in order to meet recent requirements for high image quality. The photosensitive member should be made slim and the movement distance of carriers should be reduced for preventing the deterioration of image quality. To that end, recent photosensitive members are made thinner. However, the photosensitive member is worn, for example, worn away by a cleaning every image formation process. Thus, the life of a thinner photosensitive member expires after fewer image formation processes. This shortens the life of the photosensitive member.
A third factor is an increase in importance of photosensitive member along with rise in popularity of color image. In recent years, a popularity of a color image has risen on the market from the viewpoint of facilitating the understanding of information. Incidentally, as for a color image, a photographic image or a graphic image that would occupy a wide area of a recording sheet is printed out in many cases unlike a monochrome image which mainly includes character information. In addition, the background includes solid fills in many cases. Thus, an image formation area per image formation process increases, and accordingly, an image forming unit including the photosensitive member is worn more and more. Meanwhile, an image forming apparatus of a revolver type where plural developing devices correspond to one photosensitive member, for example, has been known. Such an image forming apparatus has been widely available on the market because the number of parts can be reduced and a color image can be formed with relatively low cost. However, latent images are developed onto the photosensitive member by plural developing devices for forming one color image, so the photosensitive member is worn at a speed several times higher than the developing devices. In the above image forming apparatus, a wearing of the photosensitive member accompanying the formation of color images is particularly conspicuous. As described above, the formation of color images is one of the factors that shorten the photosensitive member's life.
Based on the foregoing three factors of the downsizing, the high-quality image formation, and the color image formation, it is supposed that the photosensitive member be worn at higher speed than the other image forming unit. Needless to say, studies about how to improve the durability of the photosensitive member and how to lengthen its usable life have been under progress, but similar studies have been made on the other image forming unit. This means that the photosensitive member's life tends to become shorter than that of the other image forming unit.
This leads to an unbalanced relation in usable lifetime between the process cartridge or photosensitive member incorporated in the image forming unit and the other image forming mechanisms. That is, the conventional process cartridge or image forming unit faces a problem in that the process cartridge should be replaced with reference to an image forming mechanism having the shortest usable life. According as the photosensitive member's life is shortened, the above problem becomes noticeable. Hence, the other image forming mechanisms should be disadvantageously replaced with reference to the short-life photosensitive member. As regards the image forming unit, the unit is replaced with reference to a photosensitive member having the shortest life among the plural photosensitive members.
In such a circumstance, a serviceable image forming unit should be discarded or recycled to impose an economical burden on a user, wastefully expend a manufacturer's effort to recover units, and adversely affect the environment.
There has been proposed an image forming apparatus where a short-life component out of the components of image forming mechanisms is preferentially replaced to reduce the burdens on the user, the manufacturer, and the environment with a view to overcoming the drawbacks resulting from the recent market requirements. According to the image forming apparatus, the plural photosensitive members can be individually and replaced as a photosensitive cassette by a user. Hence, the apparatus can deal with even such a situation that a replacement timing varies between the photosensitive members.